Why is HBrO4 a stronger acid than HBrO3?
A. Electronegativity increases when more oxygen atoms are present
B. Br is in a higher oxidation state in HBrO4
C. The bond enthalpy is higher in HBrO3
D. The Ka value is higher in HBrO3
The correct answer and explanation is :
The correct answer is B. Br is in a higher oxidation state in HBrO4.
Explanation:
The acid strength of oxoacids, such as HBrO4 and HBrO3, is largely determined by the number of oxygen atoms attached to the central atom (in this case, bromine, Br) and the oxidation state of the central atom. In oxoacids, the higher the oxidation state of the central atom, the stronger the acid. This is because a higher oxidation state increases the polarity of the O-H bond, making it easier for the hydrogen ion (H⁺) to dissociate.
Why option B is correct:
- HBrO4 is perbromic acid, and in this molecule, bromine is in the +7 oxidation state. HBrO3, on the other hand, is bromic acid, where bromine is in the +5 oxidation state.
- As the oxidation state of bromine increases from +5 in HBrO3 to +7 in HBrO4, the electron-withdrawing effect of the bromine atom becomes stronger. This causes the O-H bond to become more polarized and facilitates the release of the hydrogen ion (H⁺), making HBrO4 a stronger acid.
- The increased oxidation state leads to a greater attraction between the bromine atom and the oxygen atoms, making it easier for the acid to donate a proton (H⁺). Therefore, HBrO4 is a stronger acid than HBrO3.
Why the other options are incorrect:
- A. Electronegativity increases when more oxygen atoms are present: While it is true that more oxygen atoms generally make the molecule more acidic (due to the greater electron-withdrawing effect of oxygen), the oxidation state of the central atom plays a more significant role in determining acid strength. In this case, HBrO4 has more oxygen than HBrO3, but the main factor is the higher oxidation state of bromine in HBrO4.
- C. The bond enthalpy is higher in HBrO3: Bond enthalpy is not the main determining factor for acid strength in this context. Instead, the ability of the acid to release a proton (H⁺) is more important. The higher oxidation state of bromine in HBrO4 makes it easier to dissociate a proton than in HBrO3.
- D. The Ka value is higher in HBrO3: This is false because the stronger acid will have a higher Ka value, not the other way around. Since HBrO4 is the stronger acid, its Ka value would be higher than that of HBrO3.
In conclusion, B is the correct answer because the higher oxidation state of bromine in HBrO4 makes it a stronger acid than HBrO3.